1. Field of the Invention
The present application relates to deposition of thin films using two different metal halide reactants. For example, aluminum fluoride thin films can be deposited by atomic layer deposition using an aluminum halide reactant and a second metal reactant, such as a titanium halide.
2. Background
Aluminum fluoride, as many other metal fluorides, is a material with a high band gap and low refractive index. Due to these properties, aluminum fluoride is an interesting optical material and has been studied in the optics community as a polarizing mirror material for vacuum ultraviolet radiation and as an antireflection coating material. AlF3 is also of interest to the semiconductor industry as it is a suitable resist material for electron beam lithography, and could be used as a p-type capping layer on top of high-k dielectric in MOS structures, such as in PMOS.
In addition, much interest in AlF3 has been shown in the lithium ion battery community, with AlF3 being used as an artificial SEI-layer to improve cathode rate capability and capacity retention in lithium ion batteries. Some thirty years ago, AlF3 was also demonstrated to show ionic conductivities of the order of 10−6 S/cm, when combined with lithium fluoride. This means that the combination of AlF3 and LiF could perhaps be used as a solid lithium ion electrolyte material.
Aluminum fluoride films are traditionally deposited by a variety of physical techniques, the most often used being thermal evaporation. In addition, sputtering methods have been studied. Many of the potential uses of aluminum fluoride, such as the optical and battery related applications, require thin and conformal films. This makes atomic layer deposition an attractive method for AlF3 depositions. Some fluorides can be deposited by ALD using HF as the fluorine source. For example, AlF3 has been deposited by ALD using trimethylaluminium (TMA) and an HF-pyridine mixture as precursors. However, HF is highly corrosive and, as such, dangerous to both the ALD equipment and its operator. Research has been carried out on safer fluorine precursors, such as TiF4 and TaF5. All these processes utilize thd-complexes as the metal precursor. A downside of using these metal containing fluorine precursors is that they can lead to large amounts of metal impurities at low deposition temperatures.